YOUNG BIRDS AND RECORDS OF THE PAST 135 



already been furnished on a previous page : this may now 

 with profit, be supplemented by what is revealed in the 

 skeleton. 



The wing of the bird is a unique structure, and all that 

 concerns us here is the matter and the manner of its be- 

 coming, so to speak, so far as we can glean from the evidence 

 of the developing skeleton, displayed during the embryonic 

 and early post-embryonic stages. But three of the 

 originally five digits of the hand now remain, and these 



D^ M'l ^« V 



WING OF A YOUNG RHEA OR SOUTH AMERICAN OSTRICH. 



have undergone profound modification to fit them to 

 support the strong quills which bear the brunt of the 

 strenuous movements during flight. So that in the adult 

 we find but two separate wrist-bones, instead of several 

 as in the reptiles, while the palm-bones have become 

 welded together to form a brooch-shaped structure. The 

 finger-joints have become reduced in number and changed 

 in shape. Of the thumb, commonly only one remains : of 

 the first finger there may be three, while the third finger 

 is represented only by a small nodule of bone. 



Some birds yet retain vestiges of a claw on the thumb, 



